Conventional processing of molten metals such as steel requires the use of much heavy equipment which is very expensive both to install and to operate. In order to produce a 3 mm thick rolled steel sheet for example, the molten steel is continuously cast into a water-cooled bottomless copper mould of a continuous casting equipment and continously withdrawn therefrom in the form of slab approximately 250 mm thick. After withdrawal the slab--still containing a liquid core--is further cooled by water-spray, then air cooled, bent to horizontal and cut to length. The solidification of molten steel begins at the outer surface of the slab at a fast rate and progresses towards the centre at a gradually reduced speed. This typical pattern of solidification produces a heterogenous crystal structure and segregation which is undesirable. At the same time, stresses develop due to shrinkage of the surface and can produce cracks which may lead to the rejection of the semi-finished or finished product. The slab may have other surface defects as well which are removed by flame or mechanical scarfing. The slab then is transferred into a continuous slab reheating furnace to be reheated to the required uniform rolling temperature. By the time the slab is discharged from the furnace it is covered with hard thick scale--a mixture of ferrous oxides--which is removed by a hydraulic scale breaker prior to rolling which is the next major step in the processing line. The descaling operation is repeated at least once more during the long rolling operation. The large thickness difference between the cast slab and the rolled sheet is not desirable but necessary in order to reduce mould wear/tonne slab produced and also to reduce scarfing and scale losses. The large reduction between the slab and the sheet requires several separate reduction steps, thus a continuous hot strip mill line contains about twelve rolling mills. The average mass of a mill may approach 1000 tonne and the mill motor capacity 5000 KW. In the rolling mill line are incorporated several hundred heavy transfer rollers, which are all driven, the majority of them individually. Much other heavy equipment is used for handling and transferring the slab and sheet between operations. After the rolled sheet passes the last mill, it is wound into a coil and transferred to a continuous pickling line, whereat it is de-coiled, guided to form several long horizontal loops of variable length, passed through hot hydrochloric acid baths, cold water-spray, post treatment tank, rinse tank, hot air dryer and a set of loops again to produce a semi-bright hot rolled sheet ready for cold rolling or surface treatment like galvanising or painting. A further complex and costly apparatus is needed for reclaiming the spent hydrochloric acid, or an equally costly system for the disposal of it. The apparatus for processing steel as described above is regarded as a modern one, yet is still one of the most expensive of any kind of processing apparatus ever used.
Simplified method and apparatus for continuously casting and rolling metal have been disclosed in U.S. Pat. No. 3,368,273 of which the present applicant was a co-applicant. In the method of that patent, molten metal is formed into streams and droplets which are cooled in a liquefied cooling medium. However, the provision of the cooling liquid necessitates a complex, and expensive, structure. The heat sink capacity of solidified drops was envisaged to be inadequate to cool the liquid steel sufficiently to form a slab suitable for direct rolling; alternatively if only droplets were produced and rolled, sealing the liquid glass at the mill rolls could not practically be solved. For these reasons, the invention of U.S. Pat. No. 3,368,273 was never realised.
There are several patents directed to the production, cooling and rolling of atomised metal particles; the technology is commonly referred to as powder metallurgy processing. For example, U.S. Pat. No. 4,114,251 teaches a process which includes atomizing molten metal in an inert atmosphere, cooling the atomized particles and then rolling the particles to form an elongated metal article. However, the process described in that patent uses convection cooling; the process is unsuitable for radiation cooling due to the large mutual shielding effects of the relatively small, closely packed, particles. The inner particles close to the center of the powder stream would remain liquid in a heat radiating column of small particles. The use of baffle 20 also excludes the possibility of any significant radiation heat exchange. Thus, while known powder metallurgy methods are used in U.S. Pat. No. 4,114,251, such methods are unsuitable for high volume steel production.
U.S. Pat. No. 4,354,987 teaches the consolidation of very fine silicon powder into shotted form. The droplets are cooled by convection in a cooling gas. The specification states: "The degree of solidification is a function of the heat transfer characteristics of the gas within the cooling tower" (ref. specification, col. 5, lines 32 to 34). In an indirect way this teches against any significant radiation heat loss. While the cooling method is suitable for silicon shot production of a relatively low throughput, the method again is unsuitable for high volume steel production.
The rate of convention cooling can be increased by increasing the velocity of the counter current gas. However, the velocity of the counter current gas must be limited to avoid exaustion, i.e. if the velocity is too great, the particles would be blown upwards. Similarly, the throughput could be increased by increasing the diameter of the cooling tower. However, it is advantageous to use a small bite angle on the roles at the bottom of the cooling tower in order to maintain substantially constant flow and to avoid arching and consequent local stopping of flow, excessive accumulation of particles and forced interuption of the process. Thus, the bite angle limits the gas flow and the size of the width of the cooling tower. In practice, the cooling tower cannot be increased effectively beyond the bite angle.
In order to obtain a practicable method and apparatus of processing metal in large commercial quantities, the metal droplets must be cooled at a high rate. With convection cooling, heat is lost from the metal droplets to the surrounding gas. However, the temperature of the surrounding gas quickly approaches the temperature of the metal droplets. The cooling gas has low thermal capacity. As the temperature differential decreases, the rate of convection cooling drops accordingly. Thus, convection cooling is effective only for a relatively short period of time when the gas first enters the cooling tower, or for powder metallurgy where the relatively small amounts of heat in the powder particles can be dissipated by convection. Unlike prior art methods, the present invention uses predominantly radiation cooling to achieve satisfactory cooling of metal droplets on a large commercial scale, typically one million tons per year or more.